1. Field of the Invention
The present invention relates to a sheet processing apparatus having a finishing function configured to fold a sheet and an image forming apparatus including such a sheet processing apparatus.
2. Description of the Related Art
A conventional sheet processing apparatus having a sheet folding function includes a sheet storing portion, a folding roller pair, and an extrusion plate, as discussed in U.S. Pat. No. 5,169,376. In the sheet processing apparatus, sheets stacked and stored in the sheet storing portion are bound at its central part. Then, the bound portion is extruded to a nip portion of the folding roller pair by the extrusion plate, so that the sheets are folded into two and discharged.
Furthermore, as discussed in U.S. Pat. No. 6,003,853, regarding a sheet bundle having a large number of sheets, a fold portion of the sheet bundle is caused to pass through a folding roller pair twice depending on the number of sheets of the sheet bundle, so that the sheet bundle can be tightly folded. This mode is referred to as a double folding. By folding the sheet bundle twice, the folds can be strengthened enough to prevent the sheet bundle from being unfolded after the folding processing. Thus, a book binding with an acceptable appearance can be obtained.
Generally, a sheet used in copying machines or printers is what is generally called a cut sheet. During a sheet manufacturing process, a fiber orientation is made along a direction in which sheet fibers are blended. It is widely recognized that a close correlation exists between the fiber orientation and ease of folding a sheet.
FIGS. 20A and 20B illustrate a relationship between the fiber orientation of a sheet and the folding direction of a sheet. In FIGS. 20A and 20B, W represents a direction of sheet fibers being blended or, in other words, the fiber orientation. FIG. 20A illustrates a case where the folding direction is orthogonal to the fiber orientation. FIG. 20B illustrates a case where the folding direction is parallel to the fiber orientation.
If the folding direction is orthogonal to the fiber orientation as illustrated in FIG. 20A, a restoring force of sheet fiber acts in a direction to open the sheet bundle. For this reason, folding is relatively not easy and, as a result, a bookbinding quality deteriorates.
On the other hand, if the folding direction is parallel to the fiber orientation as illustrated in FIG. 20B, a force that acts to open the sheet bundle is relatively not affected by the sheet fibers. Thus, it is easier to fold the sheet bundle, and a bookbinding quality is high. Even sheet bundles having the same paper type (grammage) and the same number of sheets may differ in ease of folding according to a difference in relationship between the fiber orientation and the folding direction. Thus, there arises a difference in visual quality of a sheet bundle and bookbinding quality.
In an apparatus discussed in U.S. Pat. No. 6,003,853, a folding mode is selected from single folding and double folding according to the number of sheets. For this reason, even when the fiber orientation is parallel to the folding direction and, thus, a sufficient folding quality can be obtained by single folding, double folding mode may be selected, thus requiring longer bookbinding time. On the other hand, even when the fiber orientation is orthogonal to the folding direction and, thus, double folding is desirable, single folding mode may be selected depending on the number of sheets. This results in an insufficient folding quality.